If you want to play GTA 4 + episodes well then you may need to switch over to a Quad core, that game hates any CPU less than a Triple core. How are your current framerates?
A second hand Q6600/6700 would be a good idea - or a Q9550/9650 if you can find one for a good price (bear in mind, you should be able to sell your E8500 for a pretty penny to help pay for a CPU upgrade).
As for crysis, that will need a faster GPU to play at really high framerates (the 5770 will be approximately as fast as your current card). If your budget could stretch - a 5850 would be a decent upgrade. Alternatively, you add a second 4870 in crossfireX - but you will need to check if your PSU is up to the job.